Abstract

Single crystal siliconcarbide is a chemically inert transparent material with superior oxidation-resistant properties at elevated temperatures compared to black polycrystalline siliconcarbide substrates. These improved properties make crystalline siliconcarbide a good optical sensor material for harsh environments such as combustion chambers and turbine systems. Interferometric optical sensors are orders of magnitude more sensitive than electrical sensors and are proposed for these applications. Siliconcarbide itself behaves as a Fabry-Pérot etalon eliminating the need for an external interferometer for any measurement using this siliconcarbide as a sensor. The principle of the optical sensor in this study is the temperature- and pressure-dependent refractive index of siliconcarbide, which can be used to determine the temperatures and pressures of gases that are in contact with siliconcarbide. Interference patterns produced by a siliconcarbide wafer due to multiple reflections of a helium-neon laser beam of wavelength of have been obtained at temperatures up to and pressures up to . The pattern changes for the same gas at different temperatures and pressures and for different gases at the same temperature and pressure. The refractive index at the wafer-gas interface is calculated from the interference pattern and the refractive index gradients with respect to temperature and pressure, respectively, are also determined. Decoupling temperature and pressure using these gradients and the measured reflectivity data are discussed in this paper.

Received 19 April 2007Accepted 14 August 2007Published online 11 October 2007

Acknowledgments:

This work was supported by Nuonics, Inc., and the Florida Photonics Center of Excellence at the College of Optics and Photonics in the University of Central Florida. The overall project was sponsored by S. Maley, Project Manager, U.S. Department of Energy (DOE), National Energy Technology Laboratory, Morgantown, West Virginia under Award No. DE-FC36-03NT41923, and the authors appreciate her continued support for the project. However, any opinions, findings, conclusions, or recommendations expressed herein are those of the authors and do not necessarily reflect the views of the DOE. S. Bet, a graduate student in Kar’s research group, assisted in preparing the laser-microstructured SiC sample.

Article outline:I. INTRODUCTIONII. EXPERIMENTAL PROCEDUREIII. EXPERIMENTAL RESULTS AND DISCUSSIONA. Effect of temperature and pressure on interferenceB. Effect of temperature and pressure on phase shift and refractive indexC. Determination of refractive indices of uncompressed and compressed layers1. Determination of refractive index as a function of temperature2. Determination of refractive index as a function of temperature and pressure from the experimental data using a three-layer model for interference due to multiple reflectionsD. Analysis of temperature- and pressure-dependent refractive index based on Lorentz-Lorenz modelE. Sensor response decoupling model for determining the temperature and pressure1. Step 1: Determination of temperature using an optical absorber in SiC2. Step 2: Determination of the Fresnel reflection coefficient 3. Step 3: Determination of pressureIV. COMPARISON OF THEORETICAL MODELS WITH EXPERIMENTAL DATAV. CONCLUSION